It's what the other article says, for in flight message (single tick), if the key changes it might be reencrypted with a new key if the recipient key changes.

It's what the other article says, for in flight message (single tick), if the key changes it might be reencrypted with a new key if the recipient key changes. If there's a double-tick it's too late, and it won't reencrypt. In practice, unless you have a very long monologue, only one message would "leak", not the full discussion. (after the key changes you'll get a warning if you have the settings to show cryptographic details enabled).